A boil is a painful, swollen abscess of the skin. It is usually caused by the bacteria Staphylococcus aureus, which enters a hair follicle or an oil gland duct. Although the boil feels hard to the touch, it is actually filled with pus. The accumulation of pus puts pressure on adjacent nerves and causes inflammation. Boils occur in areas that are frequently chafed, such as the neck, face, ear, and extremities; but they may occur anywhere on the body. A sty is a boil located at the hair follicle of an eyelash, and a carbuncle is a group of adjoining boils with several openings through which pus drains. Treatment of a small boil consists of keeping the area clean, avoiding irritation, and applying antibiotic ointments. Large boils, particularly on the face, can cause blood poisoning, or septicemia. They must be treated medically by draining the boil of pus and applying antibiotics.